1. Field of the Invention
The present invention relates to charged particle beam exposure systems and methods and, more particularly, to a charged particle beam exposure system and method for exposing a desired pattern on a surface of an object as a result of raster scanning of charged particle beams, while controlling each of the plurality of charged particle beams such that the charged particle beams as a whole form a beam bundle having the desired exposure pattern.
2. Description of the Related Art
The present invention uses some of the teachings of the U.S. Pat. No. 5,369,282 and the U.S. patent application Ser. No. 08/241,409 filed May 11, 1994, which are herein incorporated by reference.
With the advancement in the art of fine lithographic patterning, recent integrated circuits are formed with such a high integration density that they are now used commonly and widely in industries including computers, telecommunications, system control, and the like. Looking back at the history of dynamic random access memories, for example, it will be noted that the dynamic random memories have increased the integration density as represented in terms of storage capacity of information, from 1 Mbits to 4 Mbits, from 4 Mbits to 16 Mbits and from 16 Mbits to 64 Mbits. Currently, dynamic random access memories having a storage capacity of 256 Mbits or 1 Gbits are studied intensively. In correspondence to such an increase in the integration density, extensive studies are in progress for developing the art of so-called charged particle beam exposure that uses a charged particle beam such as an electron beam for exposing fine patterns on an object. By using such a charged particle beam, it is possible to expose a pattern having a size of 0.05 .mu.m or less, with an alignment error of 0.02 .mu.m or less.
On the other hand, conventional charged particle beam exposure systems have suffered from the problem of low throughput of exposure, and there has been a pessimistic atmosphere prevailing among those skilled in the art about the production of integrated circuits by means of such a charged particle beam exposure system. It should be noted that the conventional charged-particle-beam exposure systems have used a single charged particle beam for the exposure and it has been necessary to draw a desired pattern on the object such as a substrate by a single stroke of the charged particle beam.
On the other hand, most of such pessimistic observations addressing negative predictions about the future of charged-beam-exposure system and method, are not well founded, as is typically demonstrated by the inventors of the present invention who have succeeded in constructing a block exposure system and a BAA (blanking aperture array) exposure system that provide a throughput of as much an 1 cm.sup.2 /sec. With the high throughput of 1 cm.sup.2 /sec thus achieved, the main disadvantage of the charged-particle-beam exposure system and method is substantially eliminated. Now, it is thought that the charged-particle-beam exposure system and process are superior to any other conventional exposure systems in terms of high resolution, small alignment error, quick turn around time, and reliability.
As already noted, it in particularly essential for a charged-particle-beam exposure system to have a high exposure throughput, and block exposure process or BAA process has been developed for clearing the requirement of high exposure throughput. Hereinafter, a BAA exposure system proposed previously by the inventors of the present invention will be described briefly. For the sake of simplicity, the description hereinafter will be made for an electron beam exposure system, while the present invention is by no means limited to an electron beam exposure system but is applicable to any other charged particle beam exposure systems such as an ionic beam exposure system that uses a focused ionic beam.
In a BAA exposure system, a plurality of electron beams are produced such that the plurality of electron beams as a whole form a desired electron beam bundle with a shape corresponding to a pattern to be exposed on an object. Thereby, each of the plurality of electron beams is turned on and off individually according to the desired pattern to be exposed. Thus, each time the exposure pattern is changed, different set of electron beams are turned on. While being exposed by the electron beams on the object, which may be a substrate, the object is moved, together with a stage on which the object is supported while deflecting the electron beams back and forth by activating a deflector.
In order to produce the foregoing plurality of electron beams, the BAA exposure system employs a BAA mask that is a plate formed with a number of rectangular apertures arranged in rows and columns for shaping a single electron beam incident thereto. Each of the apertures carries a pair of electrodes on opposing edges, wherein one of the electrodes is set to a ground potential level while the other of the electrodes is supplied with a control signal that changes the level between the ground level and a predetermined energization level. In response to the energization of the electrodes on the BAA mask, the path of the electron beam through the aperture is deflected and the arrival of the electron beam upon the object is controlled accordingly. In other words, the electron beams are turned-on and off on the object in response to the control signal applied to the electrodes of the apertures on the BAA mask. It should be noted that the control signals applied to the apertures on the BAA mask represent a pattern of the electron beams produced by the BAA mask, and the control signals are changed in synchronization with a raster scanning of the surface of the object by the electron beam bundle. As a result of the raster scanning, the object is exposed along a band or zone.
In such conventional BAA exposure systems and methods, there are still various problems to be overcome, such as further improvement of the exposure throughput including improvement of data transfer rate and data compression, improvement in the precision of the exposed patterns including optimization of exposure dose and improvement of resolution when expanding exposure data into bit map data, uniform distribution of the electron beam intensity throughout the substrate, improved data processing such as expansion and transfer of the exposure dot data, positive on-off control of the electron beam, easy maintenance of the BAA mask, exposure of large diameter wafers, improvement of electron optical systems, and easy switching between a BAA exposure mode and a block exposure mode, and the like.